fix: handle providers with no available models to prevent crashes

[ahosker]

Summary

• Fix runtime crashes when providers have no available models
• Add validation in provider merge process
• Prevent empty model arrays from breaking ACP agent functionality
• Graceful degradation with error logging for debugging

Fixes

• Prevents crashes in model selection dialogs
• Handles edge cases in provider configuration merges
• Improves error handling in server API endpoints

Test

OpenCode continues to work against the earlier failed models.json (attatched)
models_broken.json

Closes #5858

[rekram1-node]

/review

[ahosker]

Committed the type safety fixes.

[rekram1-node]

/review

[github-actions]

Suggestion (not a violation): This exact logic for generating the default map is duplicated at line 1541. Consider extracting to a helper function for maintainability. That said, keeping it inline is also acceptable if you prefer the explicitness.

[ahosker]

Can do, however the style guide says.

Functions: Keep logic within a single function unless breaking it out adds clear reuse or composition benefits.

[github-actions]

lgtm

[ahosker]

Too told, too many changes since this was written.